Tailrec: drop support for polymorphic recursion.

If the method that recurses over a different type arguments,
if this method is specialised, it would lead to method not
being tail-rec anymore.

Eg:

def foo[@specialized A, @specialized B]: Unit = foo[B, A]

Author: DarkDimius

src/dotty/tools/dotc/transform/TailRec.scala

@@ -176,6 +176,7 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
         }
 
         val (prefix, call, arguments, typeArguments, symbol) = receiverArgumentsAndSymbol(tree)
+        val hasConformingTargs = (typeArguments zip methTparams).forall{x => x._1.tpe <:< x._2.tpe}
         val recv = noTailTransform(prefix)
 
         val targs = typeArguments.map(noTailTransform)
@@ -228,7 +229,8 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
 
         if (isRecursiveCall) {
           if (ctx.tailPos) {
-            if (recv eq EmptyTree) rewriteTailCall(This(enclosingClass.asClass))
+            if (!hasConformingTargs) fail("it changes type arguments on a polymorphic recursive call")
+            else if (recv eq EmptyTree) rewriteTailCall(This(enclosingClass.asClass))
             else if (receiverIsSame || receiverIsThis) rewriteTailCall(recv)
             else fail("it changes type of 'this' on a polymorphic recursive call")
           }

tests/neg/tailcall/t6574.scala

@@ -4,7 +4,7 @@ class Bad[X, Y](val v: Int) extends AnyVal {
     println("tail")
   }
 
-  @annotation.tailrec final def differentTypeArgs : Unit = {
-    {(); new Bad[String, Unit](0)}.differentTypeArgs
+  @annotation.tailrec final def differentTypeArgs : Unit = { // error
+    {(); new Bad[String, Unit](0)}.differentTypeArgs // error
   }
 }